1. Field of the Invention
The invention relates to downhole units for use in boreholes in subsurface formations.
2. Description of Related Art
When drilling holes in subsurface formations it may be necessary to utilise, usually as part of the bottom hole assembly, a downhole unit having one or more formation-engaging members which may be extended or retracted relative to the main body of the unit for engagement and disengagement with the wall of the borehole. Such units may also be required for performing operations in a already-drilled borehole. For example, the unit may be a stabilizer or may be part of a bias unit for imparting a lateral bias to the bottom hole assembly, including the drill bit, for the purposes of controlling the direction of drilling.
The downhole unit of the kind to which the present invention relates is a hydraulically operated unit comprising a main body, at least one formation-engaging member mounted on the main body for pivotal movement, about a pivot axis, outwardly and inwardly relative to the main body, a movable thrust member disposed inwardly of the formation-engaging member and movable outwardly and inwardly relative to a guide structure on the main body to transmit movement to the formation-engaging member, means for subjecting the thrust member to hydraulic pressure to effect said movement thereof, and a sealing device between the thrust member and guide structure.
The present invention is particularly, but not exclusively, applicable to modulated bias units of this kind, for use in directional drilling, where the formation-engaging member or members may be periodically extended in synchronism with rotation of the unit, and in selected phase relation thereto so that, as the bias unit rotates, each formation-engaging member is extended outwardly at a selected rotational orientation of the bias unit so as to impart a desired lateral displacement thereto as the bias unit, and the rest of the bottom hole assembly, rotates. British Patent Specifications Nos. 2259316 and 2290097 describe various features of modulated bias units of this type, and also show typical prior art arrangements for the thrust member, guide structure, and sealing device.
However, problems have been experienced with these prior art arrangements. For example, Specification No. 2259316 describes arrangements where the thrust member is in the form of a piston which is linearly slidable in a cylinder, a flexible fluid-tight seal being provided between the piston and cylinder. In the described arrangement, the thrust member itself bears against the formation. If such a thrust member were to be used with a pivoted formation-engaging member, it would be necessary for the engagement between the thrust member and formation-engaging member to be such as to accommodate relative movement between the two components. In the extremely hostile environment downhole, where the components are subjected to high temperature and pressure and to abrasion from the high pressure flow of drilling fluid, rapid wear of the engaging parts of the thrust member and formation-engaging member would occur leading to reduced effectiveness of the operation of the components and ultimately failure.
British Specification No. 2259316 discloses further arrangements where the formation-engaging member is pivoted, and in such cases the thrust member is integral with the formation-engaging member or is rigidly bolted to it. Consequently, as the pivoted formation-engaging member is extended or retracted the thrust member tilts and moves laterally relative to its guide structure, and the sealing device between the thrust member and guide structure must therefore be such as to accommodate such movement.
The most successful arrangement hitherto has been to provide a flexible rolling diaphragm having an annular portion of U-shaped cross-section connected between the outer surface of the thrust member and the surrounding inner surface of the guide structure. However, in a modulated bias unit the rolling diaphragm is subject to repeated flexing movements during each rotation of the bias unit with the result that, in the hostile downhole environment, rapid deterioration of the diaphragm can occur. This problem is exacerbated by the entrapment of abrasive particles from the drilling fluid in the folds of the rolling diaphragm, which may lead to very rapid abrasive wear and ultimately failure of the seal. In an endeavour to reduce this effect, it has been proposed, as described in British Patent Specification No. 2290097, to provide a further flexible annular diaphragm connected between the movable thrust member and the surrounding wall of the guide structure outwardly of said rolling diaphragm, to shield the rolling diaphragm from debris in the drilling fluid flowing past the bias unit. However, this arrangement has not proved entirely satisfactory, and does not, in any case, have any effect on the liability of the rolling diaphragm to fail as a result of its continual cyclic flexing when the bias unit is in use.
The present invention therefore sets out to provide an improved arrangement for alleviating or overcoming the above problems, as well as providing other advantages. Although the invention is particularly applicable to bias units, and more particularly to modulated bias units, it may also be of use in any form of downhole unit of the kind referred to above, having extendable formation-engaging members which are hydraulically actuated.